* The US has unusual size, both geographically (which matters here) and population
* The US has unusual diversity
* The US has an unusually toxic history with racial diversity, in large part because unlike (say) Belgium, their racial history occurred almost entirely within their own borders
The argument therefore isn't that the US is distinctively racist. It's not! Japan, for instance, is far more racist: it's an overt part of the culture. No, the issue is instead that racism germinated and produced toxic effects in the US to a greater degree than it did in other countries.
We're still grappling with those effects. We incarcerate a huge percentage of our population, and almost 45% of our prison population is black. The median income of a black family is just over half that of a white family. Black people are poorly represented in executive ranks. Look around you: they're poorly represented in technology as a whole! As a demographic, they get less value from the public school system than any other, and so we have persistent pipeline problems reinforcing these disparities. Black people can't even reliably rent rooms on Airbnb. In 2016.
"Race realists" and "HBD people" would have you believe those phenomenon are due either to biological differences between black and white people, or to our pathological responses to those biological differences. That's horseshit, as the HBD's aptly demonstrate when they try to deploy and extrapolate from their corpus of dubious or cherry-picked statistics.
So I'd say: unless you truly believe that the black/white disparity in the US is genetic, the evidence for the US's outlier cultural issues with racism is right there in the numbers.
We're politically correct in part because we're right in the middle of trying to fix these problems.
Political correctness isn't a good thing. Also: when I take antibiotics, they wipe out my gut bacteria and give me stomach problems. That's not a good thing either! It is, in fact: a bad thing! But I don't call the whole enterprise of eradicating a bacterial infection in my body into question over it, nor do I back off the dose to try to minimize the bad side effect.